1. Field of the Invention
The present invention relates to a combustion state detecting apparatus for detecting a combustion state according to the ionic current detected from a spark plug immediately after the ignition of an internal-combustion engine and, more particularly, to a combustion state detecting apparatus for an internal-combustion engine, which apparatus changing the threshold value for shaping ionic current waveform for each ignition control and carries out statistical processing on a plurality of ionic current pulses obtained for each threshold value, thereby enabling various combustion states to be detected with high reliability.
2. Description of Related Art
In general, in an internal-combustion engine, a fuel-air mixture composed of fuel and air which has been introduced into a combustion chamber is compressed as a piston moves up, and high voltage for ignition is applied to a spark plug installed in the combustion chamber to generate an electric spark so as to burn the fuel-air mixture; the force pushing the piston down which is produced during the power stroke is taken out as a rotary output.
When the combustion takes place in the combustion chamber, the particles in the combustion chamber are ionized; therefore, applying high voltage to an ionic current detecting electrode installed in the combustion chamber causes ions with electric charges to move, thus producing a flow of ionic current.
The ionic current sensitively reacts to the combustion state in the combustion chamber, making it possible to detect a combustion state such as a misfire according to a detected ionic current value, i.e. the amount of generated ions.
The apparatus which detects a failure of normal combustion, i.e. a misfire, from the amount of ionic current detected immediately following ignition is well known (refer, for example, to the one disclosed in Japanese Unexamined Patent Publication No. 2-104978. It is also well known that, in such an apparatus, a spark plug also serves as an ionic current detecting electrode.
FIG. 7 is a block diagram illustrative of a conventional combustion state detecting apparatus for an internal-combustion engine, the apparatus employing ionic current; it shows a case wherein high voltage is distributed to the spark plugs of respective cylinders through a distributor.
FIG. 8 shows a timing chart illustrative of the operating waveforms of each signal in FIG. 7; it shows the waveforms observed when normal combustion takes place.
In FIG. 7, a crankshaft of an internal-combustion engine, i.e. an engine, which is not shown, is provided with a crank angle sensor 1. The crank angle sensor 1 issues a crank angle signal SGT composed of pulses based on the engine speed.
Each pulse edge of the crank angle signal SGT indicates the reference crank angle of each cylinder (#1 through #4) of the internal-combustion engine; the crank angle signal SGT is supplied to an ECU 2 comprised of a microcomputer and used for various control operations.
Typically, the rising edge of the crank angle signal SGT is set at the crank angle position of B75 degrees (75 degrees before the upper dead center) which corresponds to the timing at which initial energization is begun; the falling edge is set at the crank angle position of B5 degrees (5 degrees before the upper dead center) which corresponds to the timing of initial ignition.
Although it is not shown, the ECU 2 receives cylinder identifying signals generated in synchronization with engine speed in addition to the operational information from a variety of sensors. In the ECU 2, the cylinder identifying signals contribute, in association with the crank angle signal SGT, to the identification of the respective cylinders to be controlled.
The ECU 2 performs diverse control operations based on the crank angle signal SGT from the crank angle sensor 1, the cylinder identifying signals, and the operational information from the sensors, then issues driving signals for diverse actuators including an ignition coil 4.
For instance, a driving signal P for the ignition coil 4 is applied to the base of a power transistor 3 connected to a primary winding 4a of the ignition coil 4 in order to turn ON/OFF the power transistor 3, thereby turning ON/OFF the supply of primary current i1. Cutting primary current i1 off causes primary voltage V1 to rise, and secondary winding 4b of the ignition coil 4 generates further boosted secondary voltage V2 as the high voltage of a few tens of kilovolts for ignition.
A distributor 7 connected to the output terminal of a secondary winding 4b sequentially distributes and applies the secondary voltage V2 to spark plugs 8a through 8d of the respective cylinders #1 through #4 in synchronization with the revolution of the internal-combustion engine so as to burn the fuel-air mixture by generating discharge sparks in the combustion chambers of cylinders under ignition control.
A series circuit constituted by a rectifying diode D1 connected to one end of the primary winding 4a, a resistor R for limiting current, a capacitor 9 connected in parallel to a zener diode DZ for limiting voltage, and a rectifying diode D2 is connected to the ground from one end of the primary winding 4a to constitute a path for supplying charging current to a bias supply (to be discussed later) for detecting ionic current.
The capacitor 9 is charged to a predetermined bias voltage VBi of several hundreds of volts to function as a bias supply for detecting ionic current i; it supplies ionic current i by discharging via a spark plug which has just been subjected to ignition control, i.e. during the latter half period of a power stroke, among the spark plugs 8a through 8d.
A detecting resistor 10 in the path of ionic current i extending from one end of the capacitor 9 to the ground constitutes an ionic current detecting circuit for producing an ionic current detecting signal, i.e. an ionic current waveform Ei.
High-voltage diodes 11a through 11d which have the anodes thereof connected to the other end of the capacitor 9 and which are connected to the path of the ionic current i have the cathodes thereof connected to one end of the respective spark plugs 8a through 8d so that the polarities thereof are identical to ignition polarities.
The ionic current detecting signal, i.e. the ionic current waveform Ei, is compared with a predetermined threshold value TH in a comparator circuit 14 and turned into an ionic current pulse Gi which is supplied into the ECU 2 as a detected ionic current value for determining a combustion state (misfire).
Threshold value TH which provides the comparative reference for forming pulses is set to a fixed value by a line voltage which has been divided.
In FIG. 8, the ignition signal P is generated in the controlling sequence of cylinder #1, cylinder #3, cylinder #4, and cylinder #2. The primary current i1 of each cylinder is supplied to produce the secondary voltage V2. The ionic current waveform Ei is generated immediately after the cutoff timing of the ignition signal P, i.e. the ignition timing; it indicates a peak value ip and then falls.
The ionic current pulse Gi rises at time tu when the ionic current waveform Ei exceeds the threshold value TH and switches to ON for a section of a pulse width .tau..
Referring to FIG. 8, the operation of the conventional combustion state detecting apparatus for an internal-combustion engine shown in FIG. 7 will now be described.
Normally, the ECU 2 issues a fuel injection signal for an injector, not shown, and the ignition signal P for the power transistor 3 in accordance with the crank angle signal SGT, etc., and it turns ON/OFF the power transistor 3 by the ignition signal P to turn ON/OFF the supply the primary current i1.
When the primary current i1 is cut off, the primary winding 4a generates the boosted primary voltage V1. This causes charging current to flow via the path composed of the rectifying diode D1, the resistor R, the capacitor 9, and the rectifying diode D2, thus charging the capacitor 9. The charging of the capacitor 9 is terminated when the charging voltage of the capacitor 9 has become equal to the reverse breakdown voltage, i.e. bias voltage VBi, of the zener diode DZ.
When the primary voltage V1 is generated at the primary winding 4a, the secondary winding 4b in the ignition coil 4 generates the further secondary voltage V2 of a few tens of kilovolts which has been boosted to high voltage for ignition and applies it to the spark plugs 8a through 8d of the respective cylinders in the order of #1, #3, #4, and #2 in which they are listed via the distributor 7. This causes spark discharge to take place at the spark plugs of the cylinders to be subjected to ignition control, thereby burning fuel-air mixture to provide a torque from the power stroke.
The combustion of the fuel-air mixture generates ions in the combustion chamber of the cylinder, and the bias voltage VBi charged in the capacitor 9 causes the ionic current i to flow; e.g., when the fuel-air mixture is burned in the spark plug 8a, the ionic current i flows through the capacitor 9, the rectifying diode 11a, the spark plug 8a, the detecting resistor 10, and the capacitor 9 in the order in which they are listed.
The ionic current i is converted to voltage via the detecting resistor 10 and formed into the ionic current waveform Ei; it is further formed into the ionic current pulse Gi via the comparator circuit 14 before being supplied to the ECU 2.
The ECU 2 then determines whether the combustion state of the cylinder under ignition control is good, that is, whether a misfire has happened, mainly according to the presence of the ionic current pulse Gi and according as whether the rising timing tu and the pulse width .tau. of the ionic current pulse Gi satisfy judgment conditions.
In the case of normal combustion, the fuel-air mixture is burned only at the spark plug of the cylinder which is in the compression stroke among the spark plugs 8a to 8d of the respective cylinders. The cylinders are subjected to the ignition control in the order of cylinder #1, cylinder #3, cylinder #4, and cylinder #2 as previously mentioned.
In a four-stroke engine, the control cycle of each cylinder is repeated in the order of the induction stroke, the compression stroke, the power stroke, and the exhaust stroke, thus shifting from one stroke to another.
Thus, the ECU 2 detects the ionic current pulse Gi of a series corresponding to the respective spark plugs 8a to 8d while identifying fuel injection and the cylinders under ignition control, thereby determining the combustion state such as the presence of a misfire in each cylinder.
The pulse width .tau. of the ionic current pulse Gi, however, depends on the fixed threshold value TH since the threshold value TH in the comparator circuit 14 is set to a fixed value.
Hence, when determining the combustion state of an internal-combustion engine according to the pulse width .tau. of the ionic current pulse Gi obtained using the fixed threshold value TH, it is difficult to accurately identify the varying factors of the ionic current i indicative of the combustion state of the internal-combustion engine, the varying factors including the peak value ip, the generating period, i.e. the pulse width .tau., and the disturbance in waveform. This has also posed a problem in that it is difficult to accurately recognize the combustion state of the internal-combustion engine.